FIG. 8 illustrates schematically the well-known "peg adjustment" method for noting whether the axis of sight of the telescope of an optical instrument is parallel to the axis of any horizontal bubble level of that instrument.
In accordance with this prior art method, a standard in the form of a rigid elongated rod is positioned first at a stake A embedded in the ground (called a "peg") and thereafter at a similar ground-embedded peg B. Typically, pegs A and B are about 200 feet apart. The rod carries a scale which extends longitudinally of the rod and is calibrated in inches or other chosen increments of distance. A person holds the rod on the ground-embedded peg so that the rod extends vertically up from the ground. An optical instrument being tested, such as a transit T, first is leveled at a point C midway between pegs A and B and is used to read the scale marks on the rod at the axis of sight X of the instrument's telescope. As shown in FIG. 8, the scale reading on the rod when it is held vertical on peg A is a distance a1 up from the ground and the scale reading on the rod when it is held vertical on peg B is a distance b1 up from the ground. The difference between the readings (b1-a1) represents the difference in elevation between the respective ground locations of pegs B and A even though the optical instrument's axis of sight X might not be horizontal.
Next the optical instrument is set up and leveled close to peg A on the opposite side of it from peg B. With the calibrated rod held vertical on peg A and the eyepiece of the optical instrument within inches of the rod, a reading a2 is taken on the rod by sighting through the objective lens end of the instrument's telescope. Then the calibrated rod is moved to the now-remote peg B and held vertical on peg B while the optical instrument is used in the usual way to take a reading b2 on the rod. If the instrument's axis of sight is horizontal, the reading b2 should equal the reading a2 plus the difference in elevation between the ground locations of pegs A and B--that is, (b1-a1)+a2.
This prior art method requires at least two persons to carry it out--one to use the optical instrument and the other to hold the calibrated rod vertical, first on peg A while reading a1 is taken, then on peg B while reading b1 is taken, next on peg A again while reading a2 is taken, and finally on peg B again while reading b2 is taken.